This was one of my favorite games, for the Acorn Archimedes (once a popular computer model in the UK, but now largely forgotten).
The premise was simple: water flowed out of various tubes, and you had to make it reach the receptacle at the bottom of the screen.
The fluid physics are quite impressive for a 20 year old game. Water would flow freely (always downhill). You had a fixed number of blocks to help direct the water downwards, through various pipes and pressure sensors.
Later levels had acid instead of water (touching it would kill you), monsters to avoid, and my personal favorite, levels with blue and yellow water. You had to mix the two streams to make green water before it reached the bottom.
What made this game fun:
A good learning curve from very simple to fiendishly difficult. New game elements like remote switches were slowly introduced
Cleverly constructed puzzles
Realistic and predictable game physics
We hope to recapture some of these elements of fun in our upcoming puzzle game Flockwork!